It is known that a satellite in Earth orbit is subjected to numerous disturbances. The disturbances tend on the one hand to move the satellite relative to a set point position in its orbit and on the other hand to modify the attitude of said satellite relative to a set point attitude.
In order to maintain the satellite substantially in the set point position and in the set point attitude, it is necessary to perform orbit control and attitude control of said satellite.
Orbit control consists in limiting the variations of the orbital parameters generally expressed in terms of inclination, longitude and eccentricity of the orbit of the satellite. In the case of a satellite in GEO orbit, such as a telecommunications satellite, orbit control amounts to controlling the position of the satellite relative to the Earth and is also known as stationkeeping (S/K).
Orbit control of a satellite in GEO orbit generally employs a number of orbit control maneuvers during which thrusters of the satellite are activated. Orbit control of the satellite is performed by adjusting the thrust forces produced by said thrusters during the various orbit control maneuvers, and also by adjusting the durations of activation of said thrusters. In the conventional way, a number of orbit control maneuvers are performed:                North/South (N/S) maneuvers making it possible to control the inclination of the orbit of the satellite,        East-West (E/W) maneuvers making it possible to control the longitude of the orbit of the satellite.        
Eccentricity is generally controlled during E/W maneuvers in the case of chemical thrusters or during N/S maneuvers in the case of electrical thrusters.
A satellite frame of reference may be defined centered on the center of mass of said satellite and including three axes X, Y and Z: the axis X is parallel to a speed vector of the satellite, the axis Z is directed toward the Earth and the axis Y is orthogonal to the axes X and Z. In the satellite frame of reference, N/S maneuvers necessitate thrust forces along the axis Y while E/W maneuvers necessitate thrust forces along the axis X of the satellite frame of reference.
In the general situation, N/S and E/W maneuvers employ separate thrusters, possibly using different technologies (for example electrical for N/S maneuvers and chemical for E/W maneuvers). The thrusters used for N/S maneuvers may be mounted on movement means. Such movement means are employed to maintain the thrust directions of the thrusters aligned in the plane YZ with the center of mass of the satellite (which can vary in time as a function of the quantity of propellants in the tanks, the position/orientation of the equipment of the payload, etc.), in order to avoid producing torques liable to modify the attitude of the satellite.
The times of the orbit control maneuvers (i.e. the times of activation of the thrusters), the durations of said orbit control maneuvers (i.e. the durations of activation of the thrusters), and the thrust forces of said orbit control maneuvers constitute an orbit control maneuver plan. This maneuver plan is determined so as to minimize the consumption of the thrusters whilst maintaining the orbital parameters within predefined ranges.
Attitude control consists in controlling the orientation of the satellite, in particular relative to the Earth. When the satellite is on station in orbit, the disturbances apply torques that tend to cause said satellite to turn about its center of mass and therefore to modify the attitude of said satellite relative to the set point attitude. It is noted that the orbit control maneuvers can also apply disturbing torques if the thrust forces are not perfectly aligned with the center of mass of the satellite.
In order to maintain the satellite in the set point attitude, it is generally equipped with an angular momentum storage device. The angular momentum storage device includes for example at least three reaction wheels with rotation axes that are linearly independent. By controlling the rotation speed of said reaction wheels it is possible to create torques that oppose the disturbing torques.
Because of the cumulative effect of the disturbing torques, the rotation speeds of said reaction wheels, and therefore the stored angular momentum, tend to increase progressively. It is consequently necessary to desaturate the angular momentum storage device regularly in order to limit the speed excursion of said reaction wheels. By “desaturate” is meant applying external moments to the satellite which, when they are taken up by the angular momentum storage device, make it possible to reduce the quantity of angular momentum stored. This kind of unloading of the angular momentum storage device is known as angular momentum unloading.
Angular momentum unloading generally employs dedicated thrusters that are activated during dedicated attitude control maneuvers.
It is therefore clear that orbit control and attitude control, in particular angular momentum unloading of a satellite employ numerous different thrusters and/or numerous different maneuvers. Because it includes numerous different thrusters, the complexity and the manufacturing cost of the satellite are increased. Because numerous different maneuvers must be performed, the consumption of the thrusters is increased, which can reduce the service life of the satellite, especially in the case of chemical thrusters. Also, increasing the number of ON/OFF sequences of the thrusters has a negative effect on their service life. Moreover, the operational load of the ground segment is directly linked to the number of maneuvers. It is therefore desirable to limit their number.